Breast cancer is the most commonly diagnosed cancer and the second leading cause of cancer deaths among US women. Unfortunately, established risk factors for this disease generally are non-modifiable. Physical activity has been proposed to decrease breast cancer risk and several plausible biologic mechanisms do support this hypothesis. However, epidemiologic studies have reported highly inconsistent findings. Because case-control studies have more consistently observed inverse associations than have cohort studies, recall bias in case-control studies may account, partly, for the discrepancies. No study has assessed the extent of this possible bias. Further, if physical activity were to influence breast cancer risk via an endogenous hormonal mechanism, it should be more strongly related to breast cancers that are ER+/PR+. This question has not previously been explored. Finally, it is unclear whether lifetime physical activity is needed to decrease breast cancer risk; only one past study has examined this. To address these issues, we propose using already collected data from 39,876 women, aged greater than or equal to 45 years, in the Women's Health Study (WHS) to examine the associations of physical activity in middle-age, ascertained prospectively, with risk of all breast cancer (n=412) and of only ER+/PR+ breast cancers (n=222). Further, we propose to conduct a case-control study of these 412 women with breast cancer and 824 controls, matched to cases on age and date of randomization into the WHS, where the same information on physical activity now will be collected retrospectively. This will allow evaluation of any possible bias from case-control studies regarding the physical activity-breast cancer relation. Additionally, the case-control study will gather information on physical activity since age 12 years, allowing us to assess lifetime physical activity. This proposal, at relatively little cost, will help us better understand the physical activity-breast cancer relation.